The sensitivity of a typical communications receiver (i.e., the ability of the receiver to recover information in the presence of noise) depends, in part, on the noise figure of the receiver. This (the noise figure) is the excess noise generated by the receiver and translated to its input, as compared to the thermal noise (expressed in decibels). The noise figure of the receiver depends strongly on the performance of a low noise amplifier found at the front of the receiver.
An example of such an amplifier 10 is shown in FIG. 1. As shown, the low noise amplifier 10 includes a common-emitter gain stage that comprises an input capacitor 12 for receiving an RF signal, an inductor L.sub.e that provides emitter degeneration and helps set the input impedance R.sub.IN of the amplifier 10, and a bipolar transistor 14. In addition, the amplifier 10 may include a cascode stage for minimizing Miller capacitance and improving reverse isolation of the amplifier 10. The cascode stage comprises an AC grounding capacitor 16 and another bipolar transistor 18 that together receive a bias voltage V.sub.bias, and a load element Z.sub.C that typically includes an inductor and resistor in parallel. The amplifier 10 may also include a bypass MOS transistor 20 that can bypass the common-emitter gain stage when the RF signal is strong.
It is desirable for the input impedance of the amplifier 10 to be 50.OMEGA., both in its "normal" or "high-gain" mode (i.e., when the bypass transistor 20 is off) and in its "bypass" or "low-gain" mode (i.e., when the bypass transistor 20 is on). In the normal mode, the input impedance R.sub.IN of the amplifier 10 is determined in accordance with: EQU R.sub.IN.apprxeq.r.sub..pi. +.rho..beta.L.sub.e (1)
where r.sub..pi. is the small-signal base-emitter input resistance looking into the base of the bipolar transistor 14, .rho. is the dominant pole associated with .beta., and .beta. is the common-emitter current gain. The value of the inductor L.sub.e is selected from this equation.
There are several drawbacks to the design of this amplifier 10. For example, the required gain for the single-stage amplifier 10 is significant. Also, the inductor L.sub.e and the inductor in the load element Z.sub.C are typically off-chip, which allows these inductors to couple to input pins and destabilize the amplifier 10. Further, the amplifier 10 can be difficult to tune because the input impedance R.sub.IN, the input compression point, and the noise figure of the amplifier 10 are set by transistor geometry, inductor degeneration, and quiescent current, all of which are heavily interdependent. In addition, it is difficult to get the bypass mode input impedance, which is set by the MOS transistor 20, to match the normal mode input impedance R.sub.IN, which is set as discussed above.
Accordingly, there is a need in the art for an improved low noise amplifier.